Superstructures of oxycarbonate superconductor TlSr4-xBaxCu2(CO3)Oy (x=2
Tc=73K) were examined by electron diffraction and HRTEM techniques. Part of the crystals show a commensurate superstructure with a 6a0 periodicity due to crystallographic shears by c/2 to form -(C-C-C)-(Tl-Tl-Tl)-(C-C-C-)-(Tl-Tl-Tl)- type of sequences along the (100) direction in the basal planes. Incommensurate superstructures with longer periodicities up to 7.4a0 due to mixed intergrowths of -(C-C-C-C)- and -(C-C-C)- sequences were also observed. The average superstructure periodicity of the present specimen is around 7a0, which is relatively shorter than the value of 8a0 previously claimed by Goutenoire et al. [Physica C 210 (1993) 359] for a similar superconductor with Tc=62 K. This suggests the possibility of a close relationship between the modulation periodicity and the transition temperature in the TlSr4-xBaxCu2(CO3)Oy system with x=1-2.5. Observed structures were compared with an another type of superstructure recently discovered in HgBa2Sr2Cu2(CO3)Oy by Uehara et al. © 1993.